HVAC systems come in a variety of types, each with specific characteristics and operational constraints. The components include air handlers and HVAC control systems. Air handlers input output and return fans, and may include Variable Frequency Drives (VFD's). The system may use exhaust, return, and outside air damper(s) which can be opened or closed or placed in intermediate positions in response to variable conditions. Control systems may include air differential pressure transmitters, minimum outside air flow transmitters and other devices to implement the air handler fan tracking control strategies. The control system itself may be a pneumatic control system such as were popular in the 1950's, or may be a fully modern Direct Digital Control (DDC) system using controllers and network devices to implement global control of the building's pressurization and air flow.
During peak heating seasons, many multi-story buildings have difficulty maintaining comfortable space temperatures in lower floors, such as building lobby areas. Studies of these problems have often determined that the primary cause of lobby temperature issues was directly related to the invasion of cold air on lower floors as a result of “stack effect” pressure differentials exerted on the building's envelop as outside air temperatures drop below 25 F. “Stack effect” forces are described, for example, in Canadian Building Digest, Article CBD-104, “Stack Effect in Buildings” (incorporated by reference herein) and the University of Hong Kong Lecture entitled “Air Movement and Natural Ventilation”. The former details how stack effect forces are created and calculated, and the latter discusses how stack effect forces affect a building's envelop air infiltration rates and presents calculations to predict air movement.
All multi-story buildings above four stories experience building pressurization as a function of the difference between inside and outside air temperature and the resulting difference between inside and outside air density. These problems become the most extreme during the coldest winter days where the inside and outside temperatures are most divergent—building pressurization problems start becoming noticeable as outdoor air temperatures fall below 25 F. At this temperature range, “stack effect” forces created by different air densities of the outside and inside air become disruptive of the HVAC control system strategy used to control air handler fan tracking and building pressurization control.
The pressurization of a building depends on many factors including the building's height and architectural and mechanical system designs. In many cases, the most significant issue is control of HVAC mechanical systems fans, outdoor air intake and exhaust systems. Traditionally speaking, standard HVAC controls sequence strategies fail when the structure starts to encounter significant stack effect forces because the dynamics of how air is returned back to the mechanical systems changes as stack effect forces increase.
FIG. 1 illustrates building pressurization. Each box represents a single story building 100 feet tall which maintains an inside temperature of 74 degrees. For the simplicity of modeling, each building will be modeled as having no compartments or floors to stop natural air flow inside and outside the building, and relatively equal air permeability on all floors. Furthermore, for modeling, the average pressure of the air taken over all of the walls inside the building will be assumed to be equal to the average pressure of the air taken over all of the walls outside of the building, as is the normal equilibrium condition for buildings. In such a structure, basically a very tall box with no openings, there is a “neutral plane” where the pressure inside the structure is exactly equal to the pressure outside the structure. Under the conditions described above, the neutral plane occurs exactly in the vertical middle of the building. In this idealized example, the outside air temperature does not affect the position of the “neutral plane”, however, in the real world, the neutral plane of the building could be higher or lower depending on all the other forces that may affect the pressure in the building including the fans and dampers of the HVAC system.
If the air temperature inside and outside of the building is the same, then the pressure inside and outside of the building will be the same at all heights. However, if there is a difference in temperature between the inside and inside and outside of the structure (as will typically be the case when the building is climate controlled), then there will be a difference in air density between inside and outside air and, as a result, a difference in air pressure at positions spaced vertically from the neutral plane. The average pressure inside and outside the building will remain equal, and the “neutral plane” will remain at exactly half the height of the building, however, when the air inside is less dense (when the outside air is colder) then when one moves away from the “neutral plane”, pressure changes more outside than inside, and when the air inside is more dense (when the inside air is colder) then when one moves away from the “neutral plane”, pressure changes more inside than outside.
As elaborated in the above-referenced papers, the difference in pressure a given vertical distance from the “neutral plane” can be expressed as
      p    c    =      7.6    ⁢                  ⁢          h      ⁡              (                              1                                          t                c                            +              460                                -                      1                                          t                i                            +              460                                      )            
where pc is the theoretical pressure difference due to stack effect in inches of water column, h is the distance from the neutral plane height in feet, and tc and ti are outside and inside temperatures in ° F.
For example purposes, consider the seven story building of 100 ft in height (14.28 ft per story), illustrated in FIG. 1. Inside-outside pressure difference due to “stack effect” is shown in FIG. 2. As shown in FIG. 2, when the outside air (OSA) is at 74 degrees, the same as the inside air, there is no differential pressure from inside to outside at any height. However, a substantial differential pressure (lower pressure inside at the bottom, higher pressure inside at the top) occurs at 0 degrees outside temperature, and a reverse differential pressure (lower pressure inside at the top, higher pressure inside at the bottom) sets up at 90 degrees outside temperature.
FIG. 2 illustrates that the stack effect differential pressure in the winter is over 5.4 times greater that of the summer, and opposite in direction, for the reason that the indoor-outdoor temperature difference is far greater in the winter. Further note that in the summer, the upper floors of the building are under a negative pressure while the lower floors are under a positive pressure. The opposite is true in the winter, the upper floors of the building are positive and the lower floors are negative, although the wintertime pressure difference has over five times greater magnitude than the summer pressure difference on the same floor.
The difference in pressure across a building's envelop seems insignificant at first glance, but the actual air flows that can be caused by stack effect are considerable. To demonstrate, consider a fully open lobby entryway door on the first floor of a seven story building when the outside air temperature is 0 F. From FIG. 2, we see that in this condition, all other factors being equal, the lobby's pressure is −0.114 IN WC relative to outdoor conditions. We can estimate the flow through the entry way by the equation:Q=2400A√{square root over (h)}
where Q is the air flow in cubic feet per minute, A the area in square feet and h the pressure difference in inches of water. Applying this equation to our lobby entry way at 0 F we find that a 6′8″×3′ door can move 16,206 CFM at 0.114 pressure difference across the entryway. A draft of this magnitude can overwhelm mechanical systems attempting to maintain a comfortable temperature in the lobby area, causing temperatures in the lobby to drop to unacceptably low levels in the winter, as has been frequently observed in multi-story structures.
FIG. 3 is a schematic drawing of a standard HVAC system that controls air flow. In this system, a supply fan provides supply air to the building. Supply air is typically a temperature controlled mixture of outdoor air and recycled air returned from the building, which are mixed in the mixed air plenum PL-2. The amount of outdoor air that is recirculated is a function of outdoor temperature. Typically, outdoor air is used extensively when the outdoor temperature is between about 45 and 78 degrees F. At these temperatures, the HVAC system enters a so-called Economizer mode, in which an Encomizer OA Damper D-3 is opened to permit outdoor air to enter the mixed air plenum PL-2, and the return air damper D-2 is closed to cut off the flow of return air. Outside of the Economizer mode temperature range, Economizer mode is disabled, and the economizer OA damper D-3 is closed and return air damper D-2 is opened, so that return air flows to the supply fan. Outdoor air is used sparingly at these temperatures, for the reason that the outside air is more costly to temperature control than return air from the building. However, even in extreme temperatures below 45 or above 78 degrees F., a certain amount of outside air must be drawn into the system to meet air freshness standards, which depending on occupancy and other factors can require that at least 15 to 30 percent of the air supplied to the building is fresh air. Accordingly, at such temperatures, the minimum required outdoor air is supplied to the mixed air plenum PL-2 via an injection fan which is speed controlled by an airflow sensor. A minimum outside air damper D-4 is opened in this condition.
The supply fan is typically speed controlled to provide a supply air pressure sufficient to drive air into the building. The pressure of supply air is typically detected by a pressure transmitter P-1 positioned at the supply fan outlet.
Because outside air is routinely supplied to the building, to maintain an equilibrium pressure within the building, the HVAC system must exhaust a certain amount of return air outdoors. Generally, the amount of air vented to the outdoors must be equal to the amount of outdoor air being pulled into the mixed air plenum PL-22 and subsequently delivered to the building via the supply fan. The HVAC system provides this relief air path via a return/relief air plenum PL-1, which receives return air from the building, and is connected on the one hand to the mixed air plenum PL-2 for delivery of return air to the supply fan, and connected on the other hand to a relief air path leading outdoors. The air flow through the relief air path is controlled by a relief damper D-1. The return air path typically also includes a return fan which has the purpose of drawing air from the building and elevating the pressure of the air supplied to the return/relief plenum PL-1 to ensure that the air will be exhausted outdoors when the relief damper D-1 is opened.
The control applied to the return fan and relief damper D-1 typically uses two differential pressure transmitters that reference atmospheric conditions to control the air handler's return air fan speed. Pressure transducer P3 senses the relative pressure between Plenum PL-1 and the outdoor air, and controls the return fan speed to provide a slightly elevated pressure in the Air Plenum PL-1, so that air will flow out the relief air path when relief air damper D-1 is opened. Pressure transducer P-2 senses the relative pressure between the building and outdoor air, and is used to control the relief air damper. Typically, when elevated building pressure is detected by transducer P-2, indicating that more outdoor air is being supplied through the supply fan than is being exhausted via the relief air path, then damper D-1 is opened to increase the exhaust air volume. In many cases there are several relief air paths each having an independent pressure transducer and independently controlled damper.
This control algorithm, while in common use, suffers from a number of inefficiencies which have been identified by the inventor, and it is an object of the invention to improve upon these existing control methods by application of principles of the present invention.